1. Field of the Invention
The present invention relates to a scroll-type compressor, and more specifically, to a crank mechanism of the scroll-type compressor.
2. Description of Related Art
In general, a scroll-type compressor includes a first scroll member and a second scroll member within a housing. The first scroll member is provided as a fixed scroll member. The second scroll member is provided as an orbital scroll member for nonrotatable, orbital movement relative to the first scroll member. The rotation of the second scroll member is prevented by a rotation preventing mechanism provided in the compressor. The first scroll member has a first end plate and a first spiral element which axially extends from the first end plate. The second scroll member has a second end plate and a second spiral element which axially extends from the second end plate. The first spiral element and the second spiral element interfit at an angular and radial offset to form a plurality of line contacts which define at least one pair of sealed-off fluid pockets. The sealed-off fluid pockets move radially inwards due to the nonrotatable, orbital movement of the second scroll member, and decrease in volume, thereby, compressing the fluid.
A ball coupling may be used as the rotation preventing mechanism for the second scroll member. A known ball coupling-type rotation preventing mechanism has a pair of plates and a plurality of balls disposed between the plates. The pair of plates have ring-like ball rolling grooves for receiving the balls on respective surfaces facing each other. One of the pair of plates is fixed to a front housing, and the other of the pair of plates is fixed to the second scroll member.
The second scroll member is driven by a drive mechanism. The drive mechanism is constructed, for example, as disclosed in JP-A 58-67903. The drive mechanism comprises a drive shaft, a crank pin provided eccentric to the drive shaft, and a driven crank mechanism, which is swingably fitted to the crank pin and rotatably held by the second scroll member. In such a drive mechanism, the driven crank mechanism is constructed so that the driven crank mechanism can be swung relative to the crank pin, and the radius of the orbital movement of the second scroll member is variable.
In the driven crank mechanism, each of the pair of plates may be formed as a plate integrally formed with the ring-like ball rolling grooves on its one surface. Hereinafter, such type plate is referred to as a "integrally formed plate".
In a known compressor, the swing angle of the driven crank mechanism is designed to be relatively large, regardless the structure of the plates of the rotation preventing mechanism. In a case where the integrally formed plates are employed for the rotation preventing mechanism, and the swing angle of the driven crank mechanism is designed relatively large, particularly when a clutch is turned on at a high speed condition, thereby starting to rotate a drive shaft, the balls are likely to roll on a central projection of a ring form of each ring-like ball rolling groove. In particular, in the driven crank mechanism, the radius of the second scroll member is likely to become smaller by an inertia of a counter weight forming the driven crank mechanism. In other words, the ball is likely to roll not along the bottom circle line of the ring-like ball rolling groove, but along a portion closer to the central projection of the ring form of the ring-like ball rolling groove. The force causing the ball to roll on the central projection becomes greater as the swing angle of the driven crank mechanism is designed to be larger. If the ball rolls on the central projection, abrasion of the ball or the plate, or both, may occur. Thus, if the swing angle of the driven crank mechanism is designed to be too large, a defect may occur on the rotation preventing mechanism.
The driven crank mechanism may have a swing angle variation allowing mechanism for maintaining a desired performance of the compressor by absorbing any dimensional variation of the scroll members. By this swing angle variation allowing mechanism, the second scroll member may be driven without departing from the first scroll member, in order to form desired sealed-off fluid pockets. However, if the allowable range of variation of the swing angle due to the swing angle variation allowing mechanism is too large, the swing angle of the driven crank mechanism itself may become too large. In such a condition, the above-described defect on the abrasion of the balls or the plates may occur.
On the contrary, if the allowable range of variation of the swing angle due to the swing angle variation allowing mechanism is too small, a variable range of the radius of the orbital movement of the second scroll member is suppressed too small, and the second scroll member may be hard to be driven along the first scroll member at a condition maintaining the necessary contact with the first scroll member. In such a condition, maintaining a desired performance of the compressor would be difficult.
Further, the swing angle variation allowing mechanism has a function for absorbing an excessive load due to excessive fluid compression or foreign material invasion. If the allowable range of variation of the swing angle due to the swing angle variation allowing mechanism is too small, the durability of the compressor to be ensured by the function of the swing angle variation allowing mechanism may decrease.